1. Field of the Invention
The present invention relates to a reactivity control rod for a core. a core of a nuclear. a nuclear reactor and a nuclear power plant.
More particularly, the present invention relates to a reactivity control rod for a core, which can elongate the lifetime of the core, a core of a nuclear reactor composed of the reactivity control rod, which can have a long lifetime. A nuclear reactor which is cooled by a liquid metal and is able to reduce scattering of the liquid metal so as to be made into a small size thereof and a nuclear power plant which comprises the nuclear reactor.
2. Description of the Related Art
A conventional liquid metal: cooled nuclear reactor with a small size. that is, a fast reactor is disclosed in U.S. Pat. No. 5,420,897.
Moreover, a conventional fast reactor has a structure for moving a neutron reflector in a vertical direction so as to adjust (control) a leakage of neutron from the core thereof, thus to compensate a change of .reactivity of the core due to a burn-up (combustion) thereof.
In the aforesaid conventional liquid metal cooled nuclear reactors, an intermediate heat exchanger is arranged in a reactor vessel. A primary coolant performs the heat exchanging operation with a secondary coolant in the intermediate heat exchanger, and the exchanged secondary coolant is circulated to a steam generator arranged outside the reactor vessel so as to generate a steam. Namely, the conventional liquid metal cooled nuclear reactor has a structure of requiring a steam generator for generating a steam, an electromagnetic pump for circulating a secondary coolant between the reactor vessel and the steam generator, and piping equipments connecting them.
An activated liquid metal such as sodium is used as each of the coolants. For this reason, the reactor vessel and a facility using the liquid metal arranged around the reactor vessel have complicated structures, so that there is the possibility that an auxiliary facility is required in preparation for a leakage of the activated liquid metal, fire caused thereby or the like.
Moreover, in the conventional liquid metal cooled nuclear reactor, the liquid metal which is easily activated, such as sodium is used as the coolant. That is, in the steam generator, the liquid metal which is easily activated reacts to water to generate a steam. For this reason, in cases where a water leakage occurs in a heating tube of the steam generator, it is difficult to avoid an occurrence of an accident caused by the reaction between the sodium and the leaked water.
The reaction between the sodium and the leaked water causes a reaction product, so that, in order to prevent the reaction product from directly being radiated, a secondary cooling system facility must be required.
In addition, a facility for housing the reaction product must be required so that there is the possibility that the reactor system, as a whole, is made into a large size thereof, and that the cost of manufacturing the reactor system is made to be increased.
Furthermore, the electromagnetic pump is arranged in a liquid metal; however, it is coaxially arranged in series on a downstream side (lower side) of the intermediate heat exchanger in view of a heat resistant characteristic of a large-sized conductive coil of the electromagnetic pump or the like. On the other hand, each of tube plates arranged above and below the intermediate heat exchanger has a structure which is easy to receive a thermal stress, and an enlargement of its diameter causes an increase of the thermal stress so that it is taken into consideration to prevent each of the tube plates from being made into a large size thereof.
As described above, in the conventional liquid metal cooled reactor, the intermediate heat exchanger and the electromagnetic pump are vertically arranged in series; for this reason, the reactor is made into a large size thereof in its height direction (in its axial direction).
The reactor with a large size in its axial direction has a structure which is easily oscillated, thereby making it unstable.
On the other hand, in a conventional neutron reflector migration type of fast reactor, when elongating the lifetime of the core thereof, it must be necessary to make long the length of fuel assembly in the core.
That is, according to the progress of combustion of the fuel assembly, a reactivity of the fuel assembly becomes negative.
Therefore, in order to offset the negative reactivity, a neutron reflector is left up from a lower portion of the core to cover the height thereof so as to improve the ability of reflecting neutron, thereby increasing a positive reactivity of the neutron reflector, so that a reactivity of the whole core of the reactor needs to be set to 0; that is, it is necessary to make the reactor operate so as to keep a combustion in a critical state.
Thus, in order to elongate an operating period of the reactor, a fuel length of the fuel assembly must be made long. Furthermore, in cases where the fuel length of the fuel assembly is made long, the reactor vessel of the reactor becomes long as a whole; as a result, there is the possibility of deteriorating the economics of the reactor. Furthermore, there are problems of causing a change of reactivity by deformation of the core in the lifetime thereof the core, an increase of pullout force of the fuel assembly.